An MRI apparatus generates images of a test subject by placing the test subject in a uniform static magnetic field generated by a magnet to irradiate electro magnetic waves to the test subject and excite nuclear spins in the test subject, and then receiving nuclear magnetic resonance signals as electromagnetic waves generated by the nuclear spins to visualize the test subject. The irradiation of the electromagnetic waves and reception of the nuclear magnetic resonance signals are performed with an RF coil which transmits and receives electromagnetic waves of radio frequency (RF).
In recent years, the static magnetic field intensity of MRI apparatuses has been increasing to 3 tesla or more. As a result, frequency of the radio frequency pulse to be irradiated to a subject is also increasing to 128 MHz or more. If the frequency of electromagnetic waves to be radiated becomes higher, its wavelength becomes shortened to a size comparable to the size of human body. As a result, in propagation of electromagnetic waves in human bodies, wave characteristics become significant, and specifically, inhomogeneity is generated in images obtained by MRI apparatuses.
In MRI apparatuses utilizing a magnetic field of 3 tesla, which have spread over many hospitals in the past several years, for example, problem of inhomogeneity of image brightness becomes significant, which is not so problematic in MRI apparatuses of 1.5 tesla conventionally used. The problem of brightness inhomogeneity becomes more serious especially when abdominal part of human body is imaged.
It has been attempted to solve the problem of image brightness inhomogeneity due to inhomogeneity of irradiated RF waves in MRI apparatuses using an ultrahigh magnetic field (3 tesla or more) by preliminarily determining an irradiation pattern (also called an excitation pattern or excitation profile) for correcting the inhomogeneity and reproducing the preliminarily determined irradiation pattern upon imaging. As techniques for reproducing a predetermined irradiation pattern, there are (1) a method called “RF shimming” (for example, Non-patent document 1: Journal of Magnetic Resonance Imaging, 12:46-67 (2000)), (2) a method called “multi-dimensional RF pulse” (for example, Non-patent document 2: Magnetic Resonance in Medicine, 54:908-917 (2005)), and so forth.
The RF shimming is a method of reproducing an irradiation pattern by using multiple coils and changing phases and magnitudes of RF waveforms applied to the coils. The multi-dimensional RF pulse method is a technique of multi-dimensional selective excitation by simultaneous irradiation of a gradient magnetic field and RF waves, in which an optimized RF waveform is obtained by calculation. This method is used not only for correcting homogeneity of irradiation pattern, but also for exciting a part of subject.
Ultrahigh magnetic field MRI apparatuses using a magnetic field of 3 tesla or more also have a problem of increase in SAR (specific absorption rate) due to the use of RF electromagnetic waves in addition to the problem of brightness inhomogeneity. When electromagnetic waves are irradiated on a human body, a part of the energy thereof may be absorbed as heat to elevate body temperature. Degree of such absorption is evaluated with a numerical value of SAR. In ultrahigh magnetic field MRI apparatuses using a magnetic field of 3 tesla or more, SAR increases compared with, for example, that observed with MRI apparatuses using a magnetic field of 1.5 tesla, even if imaging is performed under the same condition. Therefore, there is a problem that an imaging sequence safely usable with a magnetic field of 1.5 tesla may not be used with a magnetic field of 3 tesla.
In the multi-dimensional RF pulse method, since the gradient magnetic field applied simultaneously with RF pulse draws a trajectory covering a wide range of wave number space, period of irradiating the RF pulse becomes longer. Moreover, in order to correctly reproduce the irradiation pattern, it is necessary to superimpose a moiety where electromagnetic waves are weakened by the gradient magnetic field and a moiety where electromagnetic waves are strengthened by the gradient magnetic field, and therefore the method has a problem of increase of SAR in principle.
Several techniques for reducing SAR have also been developed so far. An example is the VERSE (variable-rate selective excitation) method (Patent document 1: U.S. Pat. No. 4,760,336).
It has been so far attempted to apply the VERSE method to the multi-dimensional RF pulse method described above. However, such attempt has problems that (1) calculation of RF waveform and gradient magnetic field waveform becomes complicated, (2) since period of pulse irradiation becomes longer, error of irradiation pattern increases, and so forth.
Meanwhile, since an irradiation pattern is generally described with complex number for each sampling space element, a complex number for each point has an absolute value component and a phase component. In the method described in Non-patent document 3 (Proc. Intl. Soc. Mag. Reson. Med., 15, p 1693 (2007)), it is attempted to take only the absolute values of a correcting irradiation pattern into consideration, and use “arbitrary” values from 0 to 360 degrees for the phase components. In this method, since there is no restriction for the phase portion at all, the solution which should be calculated in minimization becomes unstable, and therefore the method has a serious drawback that a special method is required for calculation. Moreover, an RF waveform, which makes excited spin phase change rapidly along with the space sampling points, may be generated. When phase change is extremely large, phase change in one pixel may become too large to be ignored, and the pixel intensity represented as the sum may also be reduced.